Owner's Problem Description
The owner reported that after replacing the cylinder head (GMC#24573087 on GMC#24574468), the engine began to run poorly. The first and third cylinders are not functioning, as determined by inspecting the spark plugs. Despite having a spark, the cylinders are not performing. Additionally, the engine stalls when the brake pedal is pressed sharply, but runs normally with smooth braking. The owner also noted an increase in fuel consumption.
Professional Technical Analysis
The vehicle in question is a 1993 Oldsmobile Achieva with an automatic transaxle. Following a head cylinder replacement, the owner observed that the first and third cylinders were not functioning properly, despite having spark. This suggests a potential issue with either fuel delivery, compression, or possibly a mechanical fault after the head installation. After further investigation, the owner realized that the problem was traced back to the distributor cap and high-voltage wires, which were replaced, resulting in all cylinders functioning correctly. However, a new issue has emerged whereby the engine stalls under hard braking, while a gentle brake application does not induce stalling. This symptom suggests a possible vacuum leak or an issue with the torque converter not locking up correctly, which could lead to an abrupt engine load change when braking. Additionally, the increased fuel consumption could be indicative of the engine running inefficiently or in a richer condition due to improper air-fuel mixture or timing issues. A thorough check of the engine’s compression in all cylinders is recommended (normal range should be 120-150 psi), as well as an inspection of the brake booster, vacuum lines, and transmission engagement to rule out any cascading failures that could affect engine performance during braking.
Possible Causes
Most common causes (ordered by frequency):
- Vacuum Leak in Brake Booster: A vacuum leak in the brake booster can cause erratic engine performance under heavy braking conditions due to sudden changes in intake manifold pressure. This might lead to stalling when the brake is abruptly applied, as the engine receives an unintended lean condition. The brake booster should hold a vacuum of 18-22 inHg, and any leaks should be detected and repaired. - Stalling engine during hard braking, normal operation during gentle braking, vacuum gauge indicates low readings.
- Torque Converter Lockup Failure: The torque converter may not be locking up correctly, causing the engine to stall under sudden load changes such as hard braking. This can lead to a lack of engine power transfer to the wheels and stalling. The torque converter should show a lockup condition at cruising speeds and should be checked for proper operation. - Engine stalls when brake pedal is pressed hard, normal operation during smooth braking, potential slipping during acceleration.
- Fuel System Inefficiency: Given the increase in fuel consumption, the fuel system may not be delivering the correct amount of fuel to the engine, causing it to run rich. This might occur due to a malfunctioning fuel pressure regulator or clogged fuel injectors. The fuel pressure should be maintained at 35-45 PSI under load. - Increased fuel consumption, potential rich running condition, fuel pressure gauge indicates irregular readings.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Visual Inspection: Start with a thorough visual inspection of the engine compartment, focusing on the brake booster and vacuum lines. Look for any signs of wear, damage, or disconnections that could lead to vacuum leaks. Use a smoke machine or carburetor cleaner to identify leaks in the vacuum system.
- STEP 2 - Brake Booster Testing: Check the brake booster operation by pumping the brake pedal several times with the engine off to deplete the vacuum. Then start the engine and observe if the brake pedal drops slightly, indicating a functioning booster. Measure the vacuum with a gauge to ensure it is within the specified range (18-22 inHg).
- STEP 3 - Torque Converter Check: Perform a road test to evaluate the torque converter’s operation under load. Monitor the RPMs and engine behavior during hard braking to assess if the converter is engaging and locking up appropriately. If the engine stalls, further diagnosis of the transmission and converter is warranted.
- STEP 4 - Fuel Pressure Testing: Connect a fuel pressure gauge at the fuel rail test port and check the pressure during engine operation. The fuel pressure should remain steady within the 35-45 PSI range. If the pressure fluctuates or drops below this range, further investigation of the fuel system components is necessary.
